This invention pertains to means for protecting thyristors in non-conducting state from transient high voltage.
It is well known that voltage supplied by power lines contains transient high voltages resulting from lightning, switching of large loads, short circuits and other causes. Such transient voltages, taking the form of very short, sharp peaks when plotted against time, are commonly referred to as spikes. These spikes, or other excessive voltages, may exceed the forward breakover voltage of non-conducting thyristor control devices, employed to selectively connect loads across the line, resulting in destruction of the device.
Because thyristors having sufficiently high breakover voltage to be immune to spikes and the like are expensive, some other form of protection is desirable. If the thyristor is inexpensive, it may be satisfactory to merely replace a destroyed thyristor. Ralph E. Clements in U.S. Pat. No. 3,603,843, issued Sept. 7, 1971, disclosed a method and means whereby a control device could be inexpensively protected by turning it on when near destructive voltages appeared on the power line.